These investigations are conceived as a long-term broadly based study of a) the maturation of mammalian lymphoid structure and function in the developing fetus; b) the stimuli for and controls over such lymphoid maturation; c) the timing of attainment of immunologic competence by the developing fetus and the cellular nature of these maturational events; d) the dependence or independence of immunogenesis on lymphogenesis; and e) the implications of the attainment of immunologic competence by the fetus on the practical aspects of fetal- maternal well-being, and on the theoretical aspects of its bearing on fundamental immunological questions. The studies will be carried out in the fetal lamb, and take advantage of the ability to perform a variety of procedures involving intrauterine surgery, such as immunization, skin grafting, thymectomy, catheterization, etc., on the fetus in utero without interruption of gestation.